Reference: How to change your picture in Teams
On your Desktop client:
To change your profile picture, click your current picture at the top right of the screen and select Change picture.
On iOS:
To change your profile picture, tap More and then tap your picture. From there, tap Edit to take or access your photo.
On Android:
To change your profile picture, tap More 
and then tap your picture. From there, tap Edit image to take or access your photo.